In recent years, various kinds of information apparatuses have been widespread, among which there exist many systems formed with apparatuses connected with each other. It is possible, for example, to connect a host computer and a disk array device to form a computer system and use the same.
In such a computer system, a host computer and a disk array device, for example, are connected by a communication cable such as an optical transmission cable. In those days, in particular, widespread is an SAN (Storage Area Network) structured to have a plurality of host computers and a plurality of disk array devices connected through an optical switch device as shown in Patent Literature 1.
In addition, when such a system with devices connected to each other as described above has a relationship of master and servant, a procedure for cutting off power of them is defined. In the above-described computer system formed of a host computer and a disk array device, it is a common practice that the disk array device should be constantly activated while the host computer is being activated. It is therefore necessary to cut off the power of the disk array device after the host computer power cut-off operation is completed.    Patent Literature 1: Japanese Patent Laying-Open No. 2006-178720.    Patent Literature 2; Japanese Patent Laying-Open No. H11-305881.
In a system in which a power cut-off procedure is defined as described above, however, there occur various kinds of problems caused by the power cut-off processing. When the power cut-off processing is manually executed, for example, since the power of the disk array device should be cut off after waiting for completion of power cut-off of the host computer, the cut-off processing takes much time. There might occur a situation that because free time is generated while other processing is executed, for example, or for other reason after power cut-off processing of the host computer, cut-off of the power of the disk array device is forgotten. As a result, wasteful power consumption is generated.
On the other hand, as a technique for automatically cutting off power of a host computer and a disk array device, there exists a system using an uninterruptible power supply (UPS) which is disclosed in Patent Literature 2. In a case of such a system, however, since it is necessary to set up a system with an uninterruptible power supply incorporated, the system structure will be complicated to require costs.
Then, when applying the above-described manual or automatic power cut-off method to the above-described SAN, the foregoing problems become more conspicuous. In other words, in a case of SAN, since it has many devices which control power cut-off and a connection relationship between a host computer and a disk array device is complicated, its power cut-off procedure will be complicated. Moreover, in a case using the above-described UPS, the necessary number of UPSs will be increased to require more costs. Then, such a problem occurs not only in the above-described computer system with a host computer and a disk array device connected but also in a system with various information apparatuses connected with each other.
An object of the present invention is to reduce power consumption at low costs in a system with apparatuses connected with each other, which is a solution to the above-described problems.